1. Field of the Invention
This invention relates to electric components formed of conductive stainless steel having high durability and low contact resistance. In particular, this invention relates to a stainless steel separator for fuel cells and a method for making the same. The invention also relates to a solid polymer fuel cell using the stainless steel separator, which may be used as a power source of an electric vehicle or as a compact distributed power source for home use, for example.
2. Description of the Related Art
Advanced countries are being strongly urged to reduce carbon dioxide emissions to prevent global warming. Thus, fuel cells not emitting carbon dioxide have been developed for environmental conservation. Fuel cells generate electricity by reaction of hydrogen with oxygen. A fuel cell basically has a sandwich structure including two separators for supplying hydrogen and oxygen, two electrodes (a fuel electrode and an air electrode), and an electrolyte membrane (ion-exchange membrane). Fuel cells are classified into phosphoric acid, molten sodium carbonate, solid electrolyte, alkaline, and solid polymer types depending on the types of electrolyte used.
Among them, solid polymer fuel cells have the following advantages compared with molten sodium carbonate and phosphoric acid fuel cells: (1) they can operate at a significantly low temperature of about 80xc2x0 C.; (2) fuel cells with light and compact main frames can be designed; and (3) the fuel cells exhibit a short transient time, high fuel efficiency, and a high output density. Thus, solid polymer fuel cells have attracted attention as power sources for electric vehicles and as compact distributed power sources for home use.
A solid polymer fuel cell generates electricity from hydrogen and oxygen through a polymer membrane, and has a structure shown in FIG. 2. A membrane-electrode assembly 1 with a thickness of several tens to several hundreds micrometers is sandwiched by separators 2 and 3 to form a unit cell which generates an electrical potential between the separators 2 and 3. The membrane-electrode assembly 1 is a composite of a polymer membrane and electrode supports on both surfaces of the polymer membrane. Each electrode support is formed of carbon cloth which supports an electrode material such as carbon black carrying a platinum catalyst. Several tens of to several hundreds of unit cells are connected in series to form a fuel cell stack.
The separators, which partition the unit cells, must function as (1) conductors carrying electrons generated; and (2) channels for oxygen (air) and hydrogen (air channels 4 and hydrogen channels 5 in FIG. 2) and channels for water and exhaust gas (air channels 4 and exhaust gas channels 5 in FIG. 2). Thus, each separator must have the following characteristics.
As the conductor, contact resistance between the separator and the electrode membrane is preferably as small as possible because the power generating efficiency of the fuel cell decreases with generation of Joule heat as the contact resistance increases. The separator, which functions as channels, requires workability to form these channels, air tightness, and corrosion resistance.
Regarding the durability of the fuel cells, the required lifetime of fuel cells for vehicles is about 5,000 hours and the required lifetime of stationary ones used as compact distributed power sources for home use is about 40,000 hours. Thus, the requirement for home use is much more severe than that for vehicles.
Solid polymer fuel cells in use include separators formed of carbonaceous materials. The carbonaceous separators have the advantage of relatively low contact resistance and no corrosion and the disadvantage of poor impact resistance, poor compactness, and high production costs for formation of channels. The high production costs are the greatest obstacle to the broad use of fuel cells. Thus, use of metallic materials, particularly stainless steels, instead of the carbonaceous materials has been attempted.
For example, Japanese Unexamined Patent Publication No. 8-180883 discloses a metal separator that can readily form a passivation film. Unfortunately, the passivation film increases the contact resistance of the separator, resulting in decreased power generating efficiency. Accordingly, the contact resistance and corrosion resistance of this metal must be improved.
Japanese Unexamined Patent Publication No. 10-228914 discloses a metallic separator plated with gold having low contact resistance to ensure high output. If the gold plated layer is thin, the plated layer inevitably has pinholes. If the gold plated layer is thick, the separator is expensive.
Japanese Unexamined Patent Publication No. 2000-277133 discloses a separator having improved contact resistance (conductivity). In this separator, carbon powder is distributed on a ferritic stainless steel substrate. However, the use of the carbon powder also causes an increase in surface treatment cost. If the surface treated separator is damaged during assembly, the corrosion resistance thereof decreases significantly.
Recently, there have been attempts to use plain stainless steel, which have not undergone surface treatment, in separators. For example, Japanese Unexamined Patent Publication Nos. 2000-239806 and 2000-294255 (corresponding to U.S. Pat. No. 3,097,689) disclose ferritic stainless steels for separators which intentionally contain Cu and Ni, and contain reduced amounts of impurities such as S, P, and N wherein C+Nxe2x89xa60.03 mass percent and 10.5 mass percentxe2x89xa6Cr+3xc3x97Moxe2x89xa643 mass percent. Japanese Unexamined Patent Publication Nos. 2000-265248 and 2000-294256 (corresponding to U.S. Pat. No. 3,097,690) discloses a ferritic stainless steel for separators containing 0.2 mass percent or less of Cu and Ni to reduce dissolution of metallic ions and reduced amounts of impurities such as S, P, and N and satisfying the relationships, C+Nxe2x89xa60.03 mass percent and 10.5 mass percent less than Cr+3xc3x97Moxe2x89xa643 mass percent.
These inventions are based on the idea that a firm passivation film is formed to suppress deterioration due to dissolved metallic ions in catalytic activity of the catalyst carried on the electrode for the purpose of suppression of an increase in contact resistance with the electrode due to the corrosion product when the stainless steel is used without treatment. Thus, the resistance itself of the stainless steel does not decrease. Furthermore, this stainless steel does not ensure generating durability over several tens of thousands of hours (resistance to a decrease in output voltage).
Since a fuel cell separator is subjected to pressing or cutting and to form gas channels, the surface state (the state of the passivation film) of the steel strip or sheet cannot be maintained after the strip or sheet is shaped into a separator or after the separator is assembled into a fuel cell. Thus, a fuel cell separator obtained by the above process must keep satisfactory characteristics.
It would accordingly be advantageous to provide a conductive stainless steel electric component which is inexpensive, has low contact resistance comparable to that of a gold plate, and has high corrosion resistance. It would also be advantageous to provide a ferritic stainless steel for solid polymer fuel cell separators. If this stainless steel having low contact resistance and high corrosion resistance is used in separators without further surface treatment, the power generating efficiency of the fuel cell does not decrease for long periods of time.
According to an aspect of the invention, a stainless steel separator for fuel cells comprises gas channels including grooves and projections for partitioning the grooves, the separator having a composition comprising about 0.03 mass percent or less of carbon; about 0.03 mass percent or less of nitrogen, the total content of carbon and nitrogen being about 0.03 mass percent or less; about 16 mass percent to about 45 mass percent chromium; about 0.5 mass percent to about 3.0 mass percent molybdenum; and the balance being iron and incidental impurities, wherein the separator has a contact resistance of about 100 mxcexa9xc2x7cm2 or less.
Preferably, the projections have an arithmetic average surface roughness Ra in the range of about 0.01 to about 1.0 xcexcm and a maximum height Ry in the range of about 0.01 to about 20 xcexcm.
Preferably, the stainless steel separator further comprises about 0.001 to about 0.1 mass percent silver.
Preferably, the stainless steel separator further comprises about 1.00 mass percent or less of silicon and about 1.00 mass percent or less of manganese.
Preferably, the stainless steel separator further comprises about 0.005 to about 0.5 mass percent vanadium.
Preferably, the stainless steel separator further comprises at least one of titanium and niobium in a total amount of about 0.01 to about 0.5 mass percent.
Preferably, the stainless steel separator is provided with a BA film having a thickness in the range of about 10 to about 300 nm on the surface of at least the projections.
According to another aspect of the invention, a method for making a stainless steel separator for fuel cells having gas channels including grooves and projections for partitioning the grooves, comprises the steps of hot-rolling a slab to form a hot-rolled sheet having a predetermined thickness, the slab comprising about 0.03 mass percent or less of carbon, about 0.03 mass percent or less of nitrogen, the total content of carbon and nitrogen being about 0.03 mass percent or less, about 16 mass percent to about 45 mass percent chromium, about 0.5 mass percent to about 3.0 mass percent molybdenum, and the balance being iron and incidental impurities; annealing and pickling the hot-rolled sheet; and cutting the hot-rolled sheet to form the stainless steel separator.
Preferably, the surface roughness of the projections is adjusted so that the arithmetic average surface roughness Ra is in the range of about 0.01 to about 1.0 xcexcm and the maximum height Ry is in the range of about 0.01 to about 20 xcexcm.
Alternatively, the method comprises the steps of hot-rolling a slab to form a hot-rolled sheet having a predetermined thickness, the slab comprising about 0.03 mass percent or less of carbon, about 0.03 mass percent or less of nitrogen, the total content of carbon and nitrogen being about 0.03 mass percent or less, about 16 mass percent to about 45 mass percent chromium, about 0.5 mass percent to about 3.0 mass percent molybdenum, and the balance being iron and incidental impurities; annealing and pickling the hot-rolled sheet; cold-rolling the hot-rolled sheet to form a cold-rolled sheet having a predetermined thickness; press-forming the cold-rolled sheet to form the stainless steel separator.
Preferably, the method further comprises the step of annealing and pickling the cold-rolled steel sheet.
Preferably, the surface roughness of the projections is adjusted so that the arithmetic average surface roughness Ra is in the range of about 0.01 to about 1.0 xcexcm and the maximum height Ry is in the range of about 0.01 to about 20 xcexcm.
Preferably, the stainless steel separator further comprises about 0.001 to about 0.1 mass percent silver.
Preferably, the surface roughness is adjusted by pickling the separator in aqua regia or an acid mixture before or after the cutting step. Alternatively, the surface roughness is adjusted during the press-forming step or by pickling the separator in aqua regia or an acid mixture before or after the press-forming step.
Preferably, the surface roughness is adjusted by the press-forming step wherein a mold used in this step has an arithmetic average surface roughness Ra in the range of about 0.01 to about 2.0 xcexcm and a maximum height Ry in the range of about 0.01 to about 50 xcexcm.
Preferably, the stainless steel separator further comprises about 1.00 mass percent or less of silicon and about 1.00 mass percent or less of manganese.
Preferably, the stainless steel separator further comprises about 0.005 to about 0.5 mass percent vanadium.
Preferably, the stainless steel separator further comprises at least one of titanium and niobium in a total amount of about 0.01 to about 0.5 mass percent.
Preferably, a BA film having a thickness in the range of about 10 to about 300 nm is formed on the surface of at least the projections of the stainless steel separator.
According to an aspect of the present invention, a solid polymer fuel cell comprises a polymer film, electrodes, and the above-mentioned separator.